For example, in the related art, a pulverized coal fired boiler includes a furnace which has a hollow shape and is installed is a vertical direction, and a plurality of combustion burners are disposed on a wall of the furnace along a circumferential direction and are arranged in a plurality of stages in an upward-downward direction. A gaseous mixture of pulverized coal (fuel) which is crushed coal and transport air (primary air) is supplied to the combustion burner, high-temperature secondary air is supplied to the combustion burner, flames are generated by blowing the gaseous mixture and the secondary gas into the furnace, and combustion gas can be generated in the furnace. In addition, a flue gas duct is connected to the upper portion of the furnace, a superheater, a repeater, an economizer, or the like for collecting heat of a flue gas is provided in the flue gas duct, water is heated by an exhaust gas generated by combustion in the furnace, and steam can be generated. In addition, a flue gas duct is connected to the flue gas duct, a denitration device, as electric dust collector, a desulfurization device, or the like is provided in the flue gas duct, and a funnel is provided on a downstream end portion of the flue gas duct.
For example, as the boiler, there are boilers which are disclosed in U.S. Pat. No. 6,994,036 and Japanese Unexamined Patent Application Publication No. 2013-103214.
In the above-described pulverized coal fired boiler, since pulverized coal which is a fuel is combusted in a furnace, popcorn ashes (massive ashes) which solid particles may be mixed into the flue gas. Since the popcorn ashes are lumps of ashes, the popcorn ashes block a screen, a denitration device, or the like provided in a flue gas duct. Accordingly, the screen is abraded and needs to be exchanged, and a maintenance cost increases. In addition, the popcorn ashes are accumulated on the screen or the denitration device, pressure loss increases, and performance decreases. Moreover, in the flue gas treatment device of Japanese Unexamined Patent Application Publication No. 2013-103214, a popcorn-ash-trapping section is provided on the downstream side of a hopper. However, there is a concern that the popcorn-ash-trapping section may be damaged by collision of the popcorn ashes due to a long-term use.
The present invention is made to solve the above-described problems, and an object thereof is to provide an exhaust duct and a boiler in which solid particles in a flue gas can be appropriately trapped.